The present invention relates to a jointing structure utilizing a gasket, and more particularly relates to such a jointing structure utilizing a gasket which is well adapted for jointing together the mutually confronting surfaces of two members which are made of materials which have different coefficients of thermal expansion, with said gasket sandwiched between said surfaces of said two members, even when a coating material is applied to one of said surfaces of said two members.
Various jointing constructions are per se known in the art for sealing between the mutually confronting surfaces of two members which are made of materials which have different coefficients of thermal expansion. A typical example of two such members with a jointing construction applied thereto is an aluminum alloy cylinder head of an internal combustion engine mounted on a cast iron cylinder block thereof with a sealing head gasket being provided therebetween, since cast iron and aluminum alloy have very different coefficients of thermal expansion. It is very desirable to take into account, in providing any such jointing construction, of the fact that quite often a coating may be applied to one of the surfaces which the gasket is required to be sandwiched between. For example, in the above specified case of an aluminum alloy cylinder head and a cast iron cylinder block of an internal combustion engine, it is per se known and per se very desirable to provide a rust preventive coating material on such a cast iron cylinder block, and in such a case it sometimes happens that the rust preventive coating material in practice gets spread over the upper surface of the cylinder block which is supposed to receive the head, due to practical factors associated with the operation of applying said rust preventive coating material. Any jointing structure is desirably required to be able to cope with such matters.
The following problem has arisen with prior art type jointing constructions for sealing between the mutually confronting surfaces of two members which are made of materials which have different coefficients of thermal expansion, when one of said mutually confronting surfaces of said two members is coated with a coating material. Such a coating material typically has an adhesive power which varies with temperature, and which typically in fact increases with temperature. Suppose that it is the member with the lower coefficient of thermal expansion, hereinafter referred to as the first member, that is the one which is coated with the coating material. Now, when the jointing construction is repeatedly heated and cooled over a long service life, a ratchet effect takes place which tends to force the gasket out from between the confronting surfaces of the two joined members, as follows. When the jointing construction is first heated up, in other words in the initial warming up phase, the coating material is not yet heated up, at least not fully, and accordingly it has a relatively low adhesive strength. Therefore, the outer portions of the gasket are carried along by the relatively greater expansion of the second member somewhat in the outward direction along the coated surface of the first member, which, relatively, allows the outer portions of the surface of the gasket confronting it to slide on it towards its outward portions. On the other hand, when after being warmed up the jointing construction is first cooled down, in other words in the initial cooling down phase, the coating material is still fully heated up, or at least has not fully cooled down, and accordingly it has a relatively high adhesive strength. At this time, therefore, said outer portions of the gasket are relatively firmly held by the adhesive power of the coating to the outer portions of the first member, and accordingly the outward portions of the other surface of the gasket tend relatively to slide on the second member also towards the outside portions thereof, as said second member relatively contracts with respect to the first member. As this process is repeated over many cycles, the gasket is progressively pulled to the outside of the jointing construction. In the extreme case, this can cause tearing of the gasket, and in any case is liable to deteriorate sealing performance. This problem is particularly troublesome in the cited case of cylinder head gaskets for use with cylinder blocks made of cast iron and cylinder heads made of aluminum alloy. A similar but converse problem would arise if it were the member with the higher coefficient of thermal expansion that was the one which was coated with the coating material.
In the prior art, countermeasures adopted to deal with this problem have included dipping of the gasket coupling surfaces into a friction reducing agent including silicon or the like, but complete prevention of gasket extrusion has not been practicable by the use of this method, which accordingly has not been entirely satisfactory.